Hydrotherapy for Cluster Headache involves the application of cold or alternating hot and cold water to modulate neural pain pathways and vascular responses. The therapeutic mechanism likely involves sympathetic nervous system modulation, vasoconstriction, and potential reduction of inflammatory mediators, which can interrupt the trigeminal-autonomic reflex and provide temporary pain relief. The precise neurophysiological mechanism is not fully elucidated but is thought to involve complex neuroendocrine and thermoreceptor-mediated responses.